A thin film transistor formed using a semiconductor film formed over an insulating surface is a semiconductor element which is essential for a semiconductor device. Since a restriction of the allowable temperature limit of a substrate is imposed on manufacture of a thin film transistor, thin film transistors having active layers including amorphous silicon which can be deposited at relatively low temperature, polysilicon which is obtained in such a manner that crystallization is performed using laser light or a catalyst element, or the like become the mainstream of transistors used for semiconductor display devices.
In recent years, a metal oxide showing semiconductor characteristics has attracted attention, which is called an oxide semiconductor as a novel semiconductor material having high mobility obtained by polysilicon and having uniform element characteristics obtained by amorphous silicon. The metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in each of which a channel formation region is formed using such a metal oxide having semiconductor characteristics have been known (Patent Documents 1 and 2).